(1) Field of the Invention
The present invention relates to sonar noise reduction. More particularly, the present invention relates to an apparatus and method of use for the passive control of flow noise from flow-induced vibration of sonar systems.
(2) Description of the Prior Art
Currently, hull-mounted and towed arrays are subject to flow noise due to turbulent wall pressure fluctuations on the arrays. Flow noise can mask an acoustic signal with the result of reducing the effectiveness of the sonar.
A primary source of flow noise is the turbulent wall pressure fluctuations on hull mounted and towed arrays at most operational speeds due to high Reynolds numbers involved. Particular methods shown to be effective for drag and flow noise reduction include riblets, large eddy breakup devices (LEBU) and the use of polymers. Since turbulent wall pressure fluctuations have been shown to scale on the mean wall shear stress; methods which lead to skin friction drag reduction would be expected to also lead to flow noise reduction. Also, methods which result in a redistribution of energy in the wavenumber frequency spectra of wall pressure fluctuations are important for reduction in direct flow noise and flow induced vibration. The low wavenumber region is of particular importance. To date, this level of control has not been widely achieved.
The implementation of turbulence control technology is closely related to the complexity and cost of execution. In particular, the control methods can be broken down into active and passive. Passive techniques are generally easier to use and less expensive.
The injection of a polymer into the turbulent boundary layer is a proven method of significant drag reduction. While this technique may work for flow noise reduction, implementation of polymer injection involves a major impact to the hull and the environment. In addition, use of this technique is constrained by the volume of polymer which can feasibly be carried onboard a vessel.
Riblets have been shown to produce a 5-10% reduction in drag and have a small effect on wall pressure fluctuations. However, it seems unlikely that riblets would be able to achieve the objectives of the present invention. Electromagnetic turbulence control is another candidate for achieving redistribution of energy. However, the cost, complexity and ship impact of implementation is excessive in comparison to the use of a LEBU.